I. Field of the Invention
The present invention relates generally to the field of wireless communications. More specifically, the present invention relates to a method for reducing call dropping rates in a wireless communication system having multiple beam communication links.
II. Related Art
There are a variety of wireless communication systems having multiple beam communication links. A satellite-based communication system is one such example. Another example is a cellular communication system. A satellite-based communication system includes one or more satellites to relay communications signals between gateways (also referred to as xe2x80x9ccommunication stationsxe2x80x9d or xe2x80x9cbase stationsxe2x80x9d) and user terminals. Gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. User terminals can be fixed or mobile, such as a mobile telephone, and positioned near a gateway or remotely located.
A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the xe2x80x9cfootprintxe2x80x9d of the satellite. The footprint of a satellite is the geographic region on the surface of the earth covered by the satellite communication system. In some satellite systems, a satellite""s footprint is geographically divided into xe2x80x9cbeams,xe2x80x9d through the use of beam forming antennas. Each beam covers a particular geographic region within a satellite""s footprint.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,174, which issued Nov. 25, 1997, entitled xe2x80x9cMethod and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,xe2x80x9d both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In communication systems employing CDMA, separate communication links are used to transmit communication signals to and from a gateway or base station in a cellular system. A forward communication link refers to communication signals originating at the gateway or base station and transmitted to a user terminal. A reverse communication link refers to communication signals originating at a user terminal and transmitted to the gateway or base station. In situations where satellite diversity is desired, the gateway establishes two or more forward links for a given user terminal, where each forward link is established on a beam from a different satellite. For example, in a two satellite diversity configuration a first forward link is established on a beam projected by a first satellite and a second forward link is established on a beam projected by a second satellite. In the above example, the user terminal receives information or data from the gateway on both the first and second beam. Satellite diversity provides increased system performance because fewer communication links or calls will likely be dropped. For example, if the beam carrying the first forward link is blocked by an obstruction (such as, a tall building), the connection between the user terminal and gateway will continue uninterrupted on the second forward link. The user will be unaware of the beam blockage. Consequently, beam source diversity is commonly desired in a multi-beam communication system.
In a satellite-based communications system where the satellites are not stationary with respect to a point on the surface of the earth, the geographic area covered by a given satellite is constantly changing. As a result, a user terminal that was at one time positioned within a particular beam of a particular satellite can at a later time be positioned within a different beam of the same satellite and/or within a different beam of a different satellite. Furthermore, because satellite communication is wireless, a user terminal is free to move about. Thus, even in systems where the satellites are stationary with respect to a point on the surface of the earth, it is likely that over time a user terminal will be covered by different beams. Consequently, if a communication link between a user terminal and a gateway is established on a first beam and the communication link is not established on other beams prior to the user terminal no longer being covered by the first beam, then, at some point, the user terminal will no longer be able to communicate with the gateway using the established communication link. As a result, an active call between the user terminal and the gateway will be dropped. Dropping calls in a communication system is a serious problem for service providers who strive to provide uninterrupted communication services. A similar call dropping problem may occur for mobile users moving around in sectored cells in terrestrial communication systems. That is, where the cells are subdivided into two or more smaller service areas which are covered at differing frequencies or using different code spaces. Here, mobile users may travel along or repeatedly cross sector boundaries within a cell, depending on such factors as cell and sector size and local physical environment.
What is, therefore, needed is a system and method for reducing call dropping rates in a multi-beam communication system. The system and method should maintain a desired level of beam source diversity to further enhance the reliability of the communication system.
In a multi-beamxe2x80x94communication system having a user terminal, a communication station for transmitting information to and receiving information from the user terminal and a plurality of beam sources, where each beam source projects a plurality of beams, and where a communication link between the user terminal and the communication station is established on one or more beams, the present invention provides a system and method for reducing call dropping rates. Furthermore, the system and method of the present invention maintain a desired level of beam source diversity.
The method according to the present invention relies on a messaging protocol between the communication station and the user terminal. Based on messages sent from the user terminal to the communication station, the communication station can determine the most desirable beam(s) on which to transmit information or data to the user terminal. The messages sent from the user terminal to the communication station contain values representing beam strengths as measured at the user terminal. The communication station uses these values to select the most desirable beams that should be used as a communication link between the communication station and the user terminal. The beams that should be used are the beams that if used will decrease call dropping rates and provide the desired level of beam source diversity.
The method according to one embodiment of the present invention includes the steps of: (1) transmitting from the communication station to the user terminal a Beam Mask Message (BMM) containing a plurality of beam identifiers, where each of the beam identifiers identifies a beam currently available to the communication station; (2) periodically measuring at the user terminal a strength of each beam identified in the BMM; (3) periodically transmitting from the user terminal to the communication station a Pilot Strength Measurement Message (PSMM) containing a plurality of beam strength values, where each beam strength value is a function of the measured strength of one of the beams identified in the BMM; (4) based on the beam strength values in the PSMM, selecting at the communication station one or more beams that should be used as a communication link between the communication station and the user terminal (i.e. the communication station selects a new active beam set); (5) at the communication station, transmitting information on all of the beams in the new active beam set; (6) transmitting from the communication station to the user terminal a Handoff Direction Message (HDM) if the one or more beams selected in step (4) are not the same one or more beams that are in the current active beam set, where the current active beam set consists of the one or more beams on which a communication link between the communication station and the user terminal is already established; and (7) receiving at the communication station a Handoff Completion Message (HCM) transmitted from the user terminal after the user terminal receives information on each of the beams in the new active beam set.
Based on the HDM, the user terminal can determine the one or more beams that the communication station selected in step (4) that should be used as a communication link between the communication station and the user terminal. In one embodiment, the HDM includes a beam identifier corresponding to each beam selected by the communication station in step (4). In another embodiment, the HDM includes an add beam set and a drop beam set. The add beam set includes a beam identifier for each beam within the new active beam set that is not in the current active beam set. The drop beam set includes a beam identifier for each beam in the current active beam set that is not in the new active beam set.
According to one embodiment, the plurality of beam strength values included in the PSMM include a plurality of values corresponding to a strongest beam in each satellite identified in the BMM. In another embodiment, the beam strength values in the PSMM are adjusted beam strength values.
In one embodiment, the step of selecting at the communication station one or more beams that should be used as a communication link between the communication station and the user terminal includes the steps of: (1) selecting the strongest beam in the PSMM; (2) determining the strongest alternate beam in the PSMM, where an alternate beam is a beam projected by a satellite other than the satellite that projects the beam selected in step (1); and (3) selecting the strongest alternate beam in the PSMM if the strength of the strongest beam in the PSMM minus the strength of the strongest alternate beam in the PSMM is less than a threshold amount.
In another embodiment, the step of selecting one or more beams on which to establish a communication link includes the steps of: (1) selecting the strongest beam in the PSMM; (2) determining the strongest alternate beam in the PSMM; (3) selecting the strongest alternate beam in the PSMM if the strength of the strongest beam in the PSMM minus the strength of the strongest alternate beam in the PSMM is less than or equal to a first threshold amount; (4) if the strength of the strongest beam in the PSMM minus the strength of the strongest alternate beam in the PSMM is greater than the first threshold amount, determining the strongest alternate beam in the current active set, where an alternate beam in the current active set is a beam in the current active set that is projected by a satellite other than the satellite that projects the beam selected in step (1); and (5) selecting the strongest alternate beam in the current active set if the strength of the strongest beam in the PSMM minus the strength of the strongest alternate beam in the current active set is less than or equal to a second threshold amount. In one embodiment of the present invention the second threshold amount is greater than the first threshold amount.
In another embodiment, the user terminal continually measures the beam strength of each beam in the current active set. If the beam strength of a beam in the current active set is less than the beam strength of that beam as reported in the previous PSMM by a predetermined amount and remains so over a specified interval of time, then the user terminal will transmit a new PSMM to the communication station.